1. Field of the Invention
This invention relates to a conveyor oven for cooking food products, and in particular to a conveyor oven utilizing heated convection air currents as a cooking source. This invention is an improvement over the ovens described in U.S. Pat. Nos. 4,471,750; 4,576,090; and 4,960,100 and the disclosures thereof are hereby incorporated by reference.
2. Description of the Prior Art
Conveyor ovens or tunnel heaters have been in use for several years primarily in the fast food industry to cook pizzas and the like. While conveyor ovens are also known for use in a variety of non-food related industries, this invention relates to the food processing industry.
In the cooking of pizza, it is necessary to apply different heating conditions to the crust and to the topping. The dough shell should be cooked thoroughly to provide a tender, crisp crust. If the heating conditions applied to the dough shell however, are applied to the topping, the cheese will in all likelihood burn, or loose topping materials may be dislodged by high velocity convection currents if the product is initially frozen. Topping materials are typically retained by the melted cheese. Therefore it is desirable to melt the cheese without subjecting the topping to the high velocity convection currents necessary to cook the crust.
In U.S. Pat. Nos. 4,377,109 and 4,471,000, hot air under pressure from a burner is introduced into nozzles which are disposed only below the product to be cooked. As the product passes along through the conveyor oven, the hot air currents directly impinge on the crust. However, the space on the conveyor between products to be cooked permits the exposed nozzles to direct heated air against the cooking chamber overhead whereupon a stream of heated air is reflected downwardly onto the surface of the product to be cooked. Therefore, the underside of the pizza is subjected to heated air at a much higher velocity and therefore higher temperature than the upper surface which contains the toppings, cheese and the like.
In U.S. Pat. Nos. 4,471,750 and 4,576,090 there are provided foraminous air plates both above and below the product to be cooked which are mutually spaced and separated by non-foraminous plates so that the product is subjected to a blanket of turbulent heated air both above and below as it passes through the foraminous plate section and then to heat under much lower velocity as it passes through the imperforate zone. Products then pass through non-discreet zones of high and relatively low convection heating. This permits moisture within the product to migrate to the surface as the product passes through the imperforate zone so that the moisture can evaporate as the product passes through the high convection heating zone.
These patents also describe a novel air flow system whereby heated air from the burner is blown through a scroll-shaped passageway and a venturi section into a plenum for increased turbulence so that air passing through the perforate sections onto the product will be in turbulent flow. The burner is disposed in front of the blower and its longitudinal axis is contained in the plane containing the axis of rotation of the blower and the upper surface of the conveyor belt, so that return air from the cooking chamber is heated by the burner and directed axially into the blower which dispenses the heated air radially, both above and below, into the respective plenums.
One problem associated with such conveyor ovens is the need to dispense heated air uniformly across the path of travel of the conveyor so that the food travelling on the conveyor will be cooked evenly. In, for example, U.S. Pat. Nos. 4,377,109 and 4,471,000, elongated slit nozzles are provided across the lower portion of the cooking area below the conveyor. In contrast, the nozzles provided in, for example, U.S. Pat. Nos. 4,591,333, 4,626,661, and 4,679,542 are individual circular nozzles with a plurality thereof being mutually spaced across the area of the cooking chamber.
Similarly, in U.S. Pat. No. 3,908,533 slit nozzles are also provided which extend transverse to the path of travel of the conveyor and in U.S. Pat. No. 4,701,340, discrete jets are described as being disposed above and below the food to be cooked at the beginning and the ending of the heating chamber with a center portion of the chamber having discrete jets below the food and steam above.
In these convection ovens typically, heated air from one or more blowers is accumulated in a plenum and disbursed into individual fingers which in turn feed the nozzles. In, for example, U.S. Pat. No. 4,576,090, the fingers slope away from the blower so that the crosssection of each finger diminishes as the distance from the blower increases. This design is an attempt to regulate the pressure behind the nozzles for uniform flow therethrough. In U.S. Pat. No. 5,231,920, individual finger elements are described which are constant in cross-section but are provided with a pair of baffle plates within each finger. An upstream baffle plate is provided adjacent the entrance to the finger disposed in an acute angle to the flow therethrough and a downstream baffle plate is provided parallel to the longitudinal axis of the individual finger. This design also is an attempt to provide a constant velocity through nozzles along the length of the finger.
In conveyor ovens however, the design of the nozzles and the fingers and in fact the upstream plenum may account for a wide variation in blower capacity in order to achieve optimum cooking conditions. In other words, because of a torturous path required for the heated air to travel from the blower to exit the nozzles in a uniform flow, an over pressure may be required. This can be a disadvantage in that the blowers are quite noisy and excess heated air often is released into the surrounding atmosphere.
Typically in a fast food environment, it will be desired to provide a plurality of such ovens, preferably stacked, and each oven may have for example three blowers. Of necessity, the entrance and exit from each oven is open and therefore, an over designed blower system will produce a great deal of unnecessary noise and excess heat. In addition, as will be obvious to those skilled in the art, an over designed blower and heating system will be more expensive to operate, in addition to creating an undesirable atmosphere for the food handlers.